The PRML initials (Partial-Response signaling with Maximum-Likelihood sequence detection) are used to identify a technique for processing data read on a magnetic support (such as a hard disk or a magnetic tape), that provides for the use and sampling of only a part of the input signal's frequency band and the search for the maximum likelihood of correspondence of the levels of the output signal with the samples obtained from the input signal.
In a known PRML reading/writing channel the analog signal from the magnetic head is processed and reconstructed digitally through an apparatus that carries out a first analog equalization by means of a variable-gain input amplifier and a continuous low-pass filter, at whose output, with a suitable selection of the cut-off frequency, of the high-frequency accentuation (boost) and of other parameters such as group delay, it is possible to obtain a signal with a minimum InterSymbol Interference, ISI. The output of the analog filter is then sampled through an analog-digital converter and subsequently processed through a transversal digital filter (called FIR, Finite Impulse Response), that further reduces the ISI, possibly another filter (Pole Tip Filter) that eliminates the small undesired peaks produced when the magnetic support enters or leaves the influence area of a thin-film reading head, a Viterbi detector (that is, a maximum likelihood sequence detector operating with the so-called Viterbi algorithm, for example described in G. D. Forney, Jr., "The Viterbi Algorithm", Proc. IEEE 61(3):268-278, March 1973), that converts a sequence with many bits into a sequence with just one bit on the basis of a maximum likelihood concept) and a decoder capable of converting the RLL (Run Length Limited) code commonly utilized for writing data on a magnetic support in the NRZ (Non-Return to Zero) code commonly utilized for using the read signal on the part of the user.
There are also two feed-back loops, of which one containing a unit for controlling the gain of the input amplifier, that allows the stabilization of the input signal's amplitude, and the other containing a unit for controlling the timing, that allows the timing signal of the analog-digital converter to be reconstructed from the data read on the magnetic support.
There are at least two problems with these known devices. A first problem is the delay (also called "latency") inherent in the timing loop and a second problem is performance of the analog-digital converter.
The timing loop contains the analog-digital converter, the digital filter and the timing control unit. If operations are carried out with samples and medium-to-high resolution filtering coefficients, the digital filter and the timing control unit, which include adders and multipliers, are necessarily pipelined (that is, operated in succession). Such pipelining often introduces delays into the timing loop.
The analog-to-digital converter often must operate at a sampling rate equal to the RLL code's bit rate of the reading channel (typically 100 MHz). This sampling rate is difficult to obtain with a low power dissipation and a high degree of accuracy.
Another problem with these known devices is that the Viterbi detector, which is a fairly complex functional unit operating at a fairly high frequency substantially equal to the RLL code's bit rate serially processes the signal samples.